Agnosticism is a philosophical position stating that there can be no proof either way that a God does exist or does not exist. Agnostics believe that there may be a god, but that he/she is unknown or inherently unknowable.

Interpretations

Agnosticism has two general meanings.

Knowledge of the divine is impossible.

This definition is not to be taken as an alternative to atheism. Atheism and theism deal with belief, while agnosticism and gnosticism deal with the basis for such belief — in particular, knowledge. For example, agnostic atheism holds that knowledge of the divine is impossible (or currently lacking) and thus belief in any gods is unjustified and illogical. On the other hand, agnostic theism is also perfectly valid. Any theist who says ‘just have faith’ is holding such a position, as they are admitting that they have no knowledge of whether a god or gods exist and yet still believe. Gnostic atheism and gnostic theism are pretty much self-explanatory given that the definition of "gnostic" is ‘believing that knowledge of the existence or nonexistence of the divine is possible or currently held’. A gnostic atheist claims that he or she knows that a god does not exist (or thinks that one day such knowledge will be obtained), and thus believes that there is no god. A gnostic theist, on the other hand, believes in a god because he or she claims to know that the god exists.

Not believing in something without evidence.

One can be agnostic about the existence of invisible immaterial pink leprechauns inside computers making them work (for example), because there is no evidence whatsoever that they exist. Assuming they do exist would be delusional. The logical default position must be that they don't exist unless evidence is found that they do. This is because the number of things we have no evidence of is essentially infinite: anyone could easily come up with hundreds of fantastical examples similar to the one above. In addition, there would remain an unending supply of ideas that no one has thought of yet. Believing in any particular such thing may not be illogical, but one simply cannot believe in everything until proven otherwise. Therefore, the default position is to not believe in anything, including gods, until or unless presented with sufficient evidence to cause one to believe. In the absence of such evidence, non-belief is the necessary default position. (The problem in this scheme is, of course, the definition of "sufficient evidence", which will differ from person to person.)

Misunderstandings

Often, people claim that they are agnostic because they lack belief in a god but don't know for sure. This is a misnomer caused by a misunderstanding of agnosticism. In this case it is used as a softened version of atheism rather than its true meaning. What such people mean to claim is that they are weak atheists, or negative atheists, who lack belief in a god. Such a description applies to people who have never heard of the idea of a god — e.g. babies, isolated tribes of people, etc. Some would argue that "agnostic" better applies to people who simply don't yet know what they believe with respect to a claim rather than "weak atheist". However, not knowing implies a lack of conviction. A belief is a conviction in the truth of something. If that conviction in the existence of god is lacking, implicit atheism at least describes such (lack of) beliefs. Furthermore, the claim of simply lacking a belief in a god does not mean they are taking some "middle ground". The fact that they lack belief in all gods they know of means that they have rejected all of them, which is in fact a position of strong atheism. In other words, people who have considered the existence of gods and have no belief that they exist are actually strong atheists with respect to those gods they have considered. The only people who are "complete" weak atheists are those with no knowledge of any supposed deities.

The problem with this misunderstanding of agnosticism is reflected in this common scenario:

Q: "Do you believe in God?"

A: "I don't know — I guess I would consider myself agnostic."

The original question, however, was not about knowledge but about belief. On the other hand, if the original question were "Is there a god?" then the respondent can logically remain agnostic on the issue, since the question deals with knowledge.

With respect to the infinitely many possible gods, implicit atheism is the only possible option, since strong atheism requires a specific disbelief in the possible deity, which in return requires specific characteristics for the deity be explained. In reference to a specific god the position of strong atheism is perfectly valid and possibly even gnostic atheism.